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Mixing Narrow and Wide Devices
The simplest SCSI configurations use only narrow devices, or only wide devices. When this is done, the correct cable is chosen, all the devices are connected in a bus topology, and the appropriate type of termination is placed at both ends of the bus. Termination is relatively straight-forward because all devices have the same width. However, there are often situations where narrow and wide devices may need to be mixed on a single SCSI channel. This has become even more of an issue in recent years, because the newest hard disks are wide only, but many other kinds of devices are still produced for the narrow interface.
The best way of mixing wide and narrow devices is to use a host adapter that has built in support for separate segments or channels for wide and narrow devices. If you use this sort of host adapter, you can effectively set up the wide and narrow devices separately, using a narrow cable and terminator for the narrow segment (or channel), and wide hardware for the wide segment (or channel). The host adapter takes care of the issues involved in mixing the devices (though you may need to help through the setting of configuration parameters). Another reason why this sort of adapter is a good idea is that narrow devices generally use single-ended signaling, while modern wide devices require LVD for maximum performance. The two cannot be mixed on the same bus segment without the hard disks dropping down to single-ended mode.
If you do not have a host adapter with separate support for narrow and wide devices, you will have to mix them on the same SCSI chain, which introduces several complications. Here are some of the issues you will need to take into account when doing this:
As you can see, mixing narrow and wide devices on the same cable can be a bit complicated. If you are having trouble getting a mixed configuration to work, try asking for help from the vendor that sold you your hardware. Whatever the problem, chances are that someone else has experienced it before you. :^)
Next: Using SCSI With IDE/ATA